In recent years, with the diffusion of an optical fiber transmission system, techniques for integrating a great number of optical devices with a high density have been required. Planar Lightwave Circuit (PLC) has been known as one of such techniques. The PLC is an optical circuit obtained by integrating optical waveguides and waveguide-type optical devices on a silicon substrate or a quartz substrate. The PLC has high productivity and reliability and is superior in the integration and functionarity. A wavelength division multiplex transmission method has been used as a method to realize an optical fiber transmission system having a high capacity. A demultiplexer and a multiplexer are used as an optical circuit to demultiplex and multiplex a plurality of optical signal having different wavelengths in a transmitter/receiver based on the wavelength division multiplex transmission method. The PLC typically includes therein a multiplexer/demultiplexer of an arrayed waveguide grating (hereinafter called Arrayed Waveguide Grating (AWG)) circuit.
FIG. 1 illustrates the structure of a conventional AWG circuit. The AWG circuit 10 is composed of: the first slab waveguide 12 connected to input waveguides 11; the second slab waveguide 14 connected to output waveguides 15; and an array waveguide 13 connecting the first slab waveguide 12 to the second slab waveguide 14. The array waveguide 13 is structured so that neighboring waveguides are arranged with a fixed difference in the light path length. The wavelength division multiplex transmission method requires about 100 array waveguides 13 in order to handle an optical signal of 32 waves for example.
Since the above-described AWG circuit 10 requires the 100 array waveguides 13 having a fixed difference in the light path length, the AWG circuit occupies a larger area in the PLC substrate when compared with other waveguide-type optical devices. To solve this, various approaches have been made in order to reduce the size of the circuit. FIG. 2 illustrates a multiplexer/demultiplexer using the conventional AWG circuit. The first slab waveguides 22a and 22b and the second slab waveguides 24a and 24b are superposed at the same portions to allow the AWG circuit 20a used as a demultiplexer and the AWG circuit 20b used as a multiplexer to be mounted on a single PLC substrate. However, the array waveguides 23a and 23b of two groups had a disadvantage that a manufacturing error for example causes a deviated center wavelength.
Another approach is that one input waveguide used for a demultiplexer and output waveguides used for a multiplexer are connected to the first slab waveguide and output waveguides used for a demultiplexer and one input waveguide used for a multiplexer are connected to the second slab waveguide to share an array waveguide (see Patent Publication 1 for example). FIG. 3 shows the structure of the conventional AWG circuit sharing the array waveguide. The AWG circuit 30 is composed of: the first slab waveguide 32 connected to an input waveguide 31a and output waveguides 35b; the second slab waveguide 34 connected to output waveguides 35a and an input waveguide 31b; and an array waveguide 33 connecting the first slab waveguide 32 to the second slab waveguide 34. This approach requires the input waveguide to be provided at the position to which light from the array waveguides to the slab waveguides focuses (i.e., the center of a plurality of output waveguides).
FIG. 4 shows the conventional relation according to which the input/output waveguide is connected to the slab waveguide. Generally, the portion at which the input/output waveguide is connected to the slab waveguide is structured so that the input waveguide has a parabolic connecting section and the output waveguide 35 has a tapered connecting section 37 in order to allow the transmission bandwidth to have an increased spectrum. However, the conventional AWG circuit does not install the parabolic connecting section and the tapered connecting section because of arranging the input waveguides among the output waveguide arranged with a predetermined interval x depending on a wavelength interval using. Thus, a disadvantage has been caused where the connecting section connecting the input/output waveguide to the slab waveguide has a transmission bandwidth having a narrow spectrum. When the parabolic connecting section and the tapered connecting section are arranged while being superposed to each other, crosstalk is caused between the input waveguide and the neighboring output waveguide. Furthermore, the removal of the input waveguide and the neighboring output waveguide also causes a lack of a channel to which a series of wavelengths are allocated, which is impractical for the operation.
It is an objective of the present invention to provide an arrayed waveguide grating circuit in which two AWG circuits are integrated while preventing the multiplexing/demultiplexing function from having a deteriorated quality.
Patent Publication 1: Japanese Patent No. 3441437
In order to achieve the objective as described above, the arrayed waveguide grating circuit according to an embodiment of the present invention includes: a first slab waveguide connected to a first input waveguide and second output waveguides at one face; a second slab waveguide connected to first output waveguides and a second input waveguide at one face; and an array waveguide that connects the other face opposed to the one face of the first slab waveguide to the other face opposed to the one face of the second slab waveguide. The first input waveguide is connected to the first slab waveguide and is positioned outside of the second output waveguides with a second interval to an outermost output waveguide among the second output waveguides connected to the one face of the first slab waveguide with a first interval depending on a wavelength. The second input waveguide is connected to the second slab waveguide and is positioned outside of the first output waveguides with a second interval to an outermost output waveguide among the first output waveguides connected to the one face of the second slab waveguide with a first interval depending on a wavelength.
The second interval can be an interval obtained by adding a half of the first interval to a positive integer of the first interval. The second interval also can be an interval 1.5 times higher than the first interval.